Hi.

I have bumped into a problem a couple of times which I can't seem to

quite understand how to handle best. It revolves around casting the

type of a generic collection.

My situation is something like this: An interface DiskResource, whose

instances users get from a DiskProvider extends Provider instance.

The DiskProvider has these two methods:

releaseDiskResource­(DiskResource diskResource);

releaseDiskResource­s(Collection<DiskRes­ource> diskResources);

I have this helper class for implementing Providers, called Helper.

This helper requires that the Resources implement Helped. I therefore

make a DiskResourceImpl which implement DiskResource and Helped. I

thus need to implement the DiskProviderImpl as such:

releaseDiskResource­(DiskResource diskResource) {

// Cast, with unchecked-warning:

helper.doInternalSt­uff((Helped) diskResource)

}

releaseDiskResource­s(Collection<DiskRes­orce> diskResources) {

// PROBLEM HERE, this cast is an ERROR, not a warning:

helper.doInternalSt­uff((Collection<Help­ed>) resourceImpls);

}

How am I supposed to do this cast? See, I know why it isn't allowed to

cast like that, but I want to nevertheless! It seems strangely

non-symmetric that I just cast it in the singular version (with a

unchecked-warning),­ but end up with an error in the other. Is the

latter that more dangerous than the first?

Both these are "solutions":

releaseDiskResource­s(Collection<DiskRes­orce> diskResources) {

Collectioc<Helped> c = new HashSet<Helped>();

for(DiskResource diskResource : diskResources) {

c.add((Helped) diskResource);

}

helper.doInternalSt­uff(c);

}

releaseDiskResource­s(Collection<DiskRes­orce> diskResources) {

helper.doInternalSt­uff((Collection) c);

}

.. of which I refuse to use #1, while #2 seems like a just-to-weird

hack around the compiler error, replacing the error with two warnings

(!). Raw types are the evilest there is, are they not?

I hope this example points out my annoyance while not being too convoluted.

Where is my thinking derailing? I have a feeling it is a couple of

steps before these methods' implementation, but I've tried different

attacks now. I don't want the users of this API to see internal stuff

that is useless for them.

Regards.

Endre.

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Your helper methods need to accept "Collection<? extends Helped>". Then you

should be fine, and you don't even need to cast.